An abstract implementation of a hash-based map that links entries to create an
ordered map and which provides numerous points for subclasses to override.

This class implements all the features necessary for a subclass linked
hash-based map. Key-value entries are stored in instances of the
LinkEntry class which can be overridden and replaced.
The iterators can similarly be replaced, without the need to replace the KeySet,
EntrySet and Values view classes.

Overridable methods are provided to change the default hashing behaviour, and
to change how entries are added to and removed from the map. Hopefully, all you
need for unusual subclasses is here.

This implementation maintains order by original insertion, but subclasses
may work differently. The OrderedMap interface is implemented
to provide access to bidirectional iteration and extra convenience methods.

The orderedMapIterator() method provides direct access to a
bidirectional iterator. The iterators from the other views can also be cast
to OrderedIterator if required.

All the available iterators can be reset back to the start by casting to
ResettableIterator and calling reset().

The implementation is also designed to be subclassed, with lots of useful
methods exposed.

mapIterator

Gets an iterator over the map.
Changes made to the iterator affect this map.

A MapIterator returns the keys in the map. It also provides convenient
methods to get the key and value, and set the value.
It avoids the need to create an entrySet/keySet/values object.
It also avoids creating the Map.Entry object.